An image sensing apparatus such as a digital camera is required to precisely remove noise components, so as to meet user's requirements for a still higher sensitivity and a larger number of pixels.
As a conventional noise reduction method, for example, Japanese Patent Laid-Open No. 2008-015741 describes a technique which divides an image into a plurality of frequency bands, removes noise components from high-frequency components, preserves edge components of the high-frequency components, and combines these high-frequency components and low-frequency components.
As a conventional signal processing method, for example, Japanese Patent Laid-Open No. 2001-045308 describes a technique for setting different gammas in luminance signal processing and chrominance signal processing.
Furthermore, as another conventional signal processing method, a technique for generating luminance signals from R, G, and B signals after gamma processing is available.
A conventional arrangement will be described below with reference to FIG. 4.
Referring to FIG. 4, input image data 3000 is RGB Bayer matrix data, and is stored in a memory 3010. The image data stored in the memory 3010 will be referred to as first-resolution image data. The input image data 3000 undergoes low-pass filter processing by an LPF 3101 and down-sampling (reduction) processing by a DS unit 3102 to generate second-resolution image data, which has a resolution lower than the first-resolution image data, and the second-resolution image data is stored in a memory 3110. At this time, the data in the memory 3110 is synchronized for respective color filters to be data in which each pixel has signals of all of R, G, and B color filters. This second-resolution image data undergoes low-pass filter processing by an LPF 3201 and down-sampling processing by a DS unit 3202 to generate third-resolution image data, which has a still lower resolution and is synchronized. The generated data is stored in a memory 3210.
An NR processing unit 3011 applies noise reduction processing to the first-resolution image data output from the memory 3010, and an edge detection unit 3012 detects edge intensities of the first-resolution image data output from the memory 3010. An NR processing unit 3111 applies noise reduction processing to the second-resolution image data output from the memory 3110, and an edge detection unit 3112 detects edge intensities of the second-resolution image data output from the memory 3110. An NR processing unit 3211 applies noise reduction processing to the third-resolution image data output from the memory 3210, and stores the image data after the noise reduction processing in a memory 3221.
A US unit 3222 applies up-sampling (enlargement) processing to the third-resolution image data stored in the memory 3221, thus generating image data having the same number of pixels as the second-resolution image data. A combining unit 3120 combines the image data output from the NR processing unit 3111 and that output from the US unit 3222 using edge intensity detection signals output from the edge detection unit 3112. More specifically, the combining unit 3120 mixes these two data while increasing a weight of the image data output from the NR processing unit 3111 for a pixel which is determined by the edge detection unit 3112 to have a large edge intensity. Conversely, the combining unit 3120 mixes these two data while increasing a weight of the image data output from the US unit 3222 for a pixel which is determined to have a small edge intensity.
The image data output from the combining unit 3120 is temporarily stored in a memory 3121, and a US unit 3122 applies up-sampling processing to the image data stored in the memory 3121, thus generating image data having the same number of pixels as the first-resolution image data. A combining unit 3020 combines the image data output from the NR processing unit 3011 and that output from the US unit 3122 using edge intensity detection signals output from the edge detection unit 3012. The combining unit 3120 then stores the combined image data in a memory 3030. More specifically, the combining unit 3020 mixes these two image data while increasing a weight of the image data output from the NR processing unit 3011 for a pixel which is determined by the edge detection unit 3012 to have a large edge intensity. Conversely, the combining unit 3020 mixes these two image data while increasing a weight of the image data output from the US unit 3122 for a pixel which is determined to have a small edge intensity.
R, G, and B signals of the image data output from the memory 3030 are respectively input to an R gamma circuit 3032, G gamma circuit 3033, and B gamma circuit 3034 since they individually undergo gamma processes. Also, chrominance signals (U, V) of the image data output from the memory 3030 are input to a chrominance signal processing circuit 3031 since they undergo chrominance signal processing. That is, the memory 3030 is required to store the R, G, and B signals for luminance signals, and the U and V signals for chrominance signals.
For this reason, the two image data input to the combining unit 3020 are required to respectively include the R, G, and B signals for luminance signals and the U and V signals for chrominance signals. Thus, since the memories 3121 and 3221 require synchronized data as both the luminance and chrominance signals, they require data for a total of five planes, that is, R, G, and B data for luminance signals and U and V data for chrominance signals, resulting in a large circuit scale. Note that the U and V signals are generated by a known method before each NR processing unit applies the noise reduction processing, and each NR processing unit applies the noise reduction processing to the R, G, B, U, and V signals.
Another conventional arrangement will be described below with reference to FIG. 5. In this example, image data output from the memory 3030 is input to a Y gamma circuit 3035 in place of the R gamma circuit 3032, G gamma circuit 3033, and B gamma circuit 3034 in FIG. 4. That is, the memory 3030 is required to store image data including Y signals for luminance signals and U and V signals for chrominance signals. For this reason, the two image data input to the combining unit 3020 need only be those each of which includes Y signals for luminance signals and U and V signals for chrominance signals. Hence, the memories 3121 and 3221 need only store data of a total of three planes, that is, Y signals for luminance signals and U and V signals for chrominance signals, thus reducing the capacities of the memories 3121 and 3221. Note that the Y, U, and V signals are generated by a known method before each NR processing unit applies the noise reduction processing, and each NR processing unit applies the noise reduction processing to the Y, U, and V signals.
In the arrangement shown in FIG. 4, the capacities of the memories 3121 and 3221 become large, resulting in a large circuit scale. In the arrangement shown in FIG. 5, which can solve this problem, since the gamma processing is applied after the Y signals are generated, image quality lowers compared to the arrangement in which luminance signals are generated from R, G, and B signals after the gamma processing.